Recovery of hydrocyanic acid



Dec. 20, 1938. F. w. sPERR, JR

RECOVERY OF HYDROCYANIVC ACIDA Filed June 25', L73?v touw@ ede Patented Dec. 20, 1938 PATENT OFFICE RECOVERY F HYDROCYANIC ACID Frederick W. Sperr, Jr., Vineland, N. J., assigner to Rhm & Haas Company, Philadelphia, Pa.

` Application June 25, 1937, serial No. 150,236

8 Claims.

` Y inga dilute solution of hydrocyanic acid containing some hydrogen sulde with a soluble nickel salt under proper conditions, all of the hydrolcyanic acid may be precipitated and by subsequent treatment of this precipitate with an acid,

the-hydrocyanic acid and residual traces of hyldrogen sulde rare set free and maybe separated vby fractional rectification in a suitable apparatus.

LThis process is carried out in two steps, the Vilrst of which comprises the preparation and separation of nickel cyanide inthe form of a iilter cake, and the second the treatment of this iilter 'cakefin a separate apparatus for the production r of substantially vpure hydrocyanic acid. It is possible to carry out the process Vcontinuously and Vthisis the preferred form, particularly in cases where it is possible to regenerate the hydrocyanic acid in the Vsame plant in which the nickel cyanide isV formed.

Most by-product coke plants use the so-called f direct process for'the manufacture of ammonium sulfatein which the gas after the removal of the water, is passed through sulfuric acid. The aml',v monium sulfate which is formed crystallizes out f and is recovered by centrifuging. 'I'he gas, which contains both hydrocyanic acid, and hydrogen sulfide, leaving .the sulfuric lacid solution is kusually at atemperature of about 50 to 60 C. This gas'. requires cooling which is generally Vaccomplished by direct scrubbing with water. yPractically all of the lhydrocyanic acid produced in 4thecoking rprocess eventually passes unchanged -through the sulfuricl acid and if suiiicientv cooling water at a suiiicientlylow temperature is em- Y ployed in a'scrubber of adequate capacity, sub- -1 rThe' object of thisinvention is to provide an vstantially all of the yhydrocyanic acid in the gas iswashed out, thus forming a very dilute solution which has hitherto, been allowed togo to waste. This solutionumay contain from 0.10 to j 0.25 gm. of hydrocyanic acid per liter and hydrogen sulphider equivalent to from to 50%fof th weight of the hydrocyanic acid present.

economical process forrecovering the hydrocy- I 4'anic acidA contained inl these dilute solutions.

r'I'he present process may be carried out in the i a 'fcc 1apparatus shown in the drawing in'which-y af, Figure yl shows an apparatus for the separation rof hydrocyanic acidas nickel cyanide;

Figure 2, an apparatus for regenerating and y collecting substantially pure hydrocyanic acid from the nickel cyanide formed in the apparatus shown in Figure 1.

If desired, most of the hydrogen sulphide in the dilute solution from the'scrubbers may be removed by aeration, as described in my copending application Serialy No. 734,420 filed Julyl 9, 1934. In this case, the dilute solution is led to the top of a cylindrical tower I, through pipe 2. Compressed air is blown into the tower at a point near the bottom through pipe 3 and distributor 4 which distributor may consist simply of a row of perforated pipes.

To provide for removal of any naphthalene scum that may accumulate at the top of the tower, pipe A with Valve B may be installed. Valve B may be opened from time to time to allow'water and scum to run oi either to waste or tothe sump which is usually provided for skimming naphthalene from gas cooler water.

To eliminate the last traces of hydrogen sulfide, a little chlorine or other oxidizing gas may be added to the air in pipe 3. I may, however, permit the small amount of hydrogen sulfide to remain and be precipitated as nickel sulfide, which will be decomposed as described below.

The waterv passes out of the bottom of the tower through pipe 8 into vertical pipe 9 having an overflow I0, the height of which is adjusted to maintain a suitable constant level in the tower I. The water flows from pipe I0 into mixing tank II equipped with agitating means I2.

A solutiony of nickel sulfate is pumped from.

tank I3 by pump I4 through pipe I5 into pipe 8 and becomes thoroughly mixed with the Water as it iiows upward through pipe 9. Milk of lime contained in tank I6 is pumped by pump I1 through pipe I8 into the mixing tank II. Ihorough mixing of the lime with the water at this point facilitates the eiiicient precipitation of nickel cyanide and its eiiicient removal in the subsequent apparatus.

In practice I have found that best conditions for the eiiicient precipitation of nickel cyanide arel obtained when the nickel sulfate solution is added at such a rate as to supply from 10 to 20% more nickel sulfate than is theoretically necessary to combine with the sum of the hydrogen sulfide and hydrocyanic acid present. The milk of lime is then added at such a rate as to maintain a pH of 6.9 to 7.6 in the water leaving mixing tank lI I. Fair results may be obtained within a pH range of 6.8 to 8.2, but out of this range the amounts of Vhydrocyanic acid and nickel lost in the nal eiliuent increase unduly. The nickel salt and lime or other alkali can be addedin any desired order. Y v

. Pumps 6, I4 and I1 may have automatic controls to regulate the rate of. chemical additions proportionately to the rate of flow of cooler water. It is better, however, to maintain a constant rate of pumping the cooler water and a constant rate of operating pump I4 handling the nickel sulfate solution, While making pumps 6 and I'I automatically responsive to variations of hydrogen ion concentration in the water.

The water flowing out of mixing tank. H contains a precipitate consisting principally of nickel` cyanide, with some nickel hydroxide, nickel carbonate and calcium carb-onate. A small amount of nickel sulde may also be present. This water flows through pipe I9 into a Dorr thickener 20. Any other efficient type of thickener may be used, the purpose being to obtain the precipitated material in the form of a fairly concentrated slurry and producing a clear eiTluent which may be run to waste or used for gas cooling or other purposes. The water with precipitate flows into feed well 2| from which it overflows into the thickener tank. The'solids drop to the bottom of the thickener and are collected in the well 22 by the action of the sweeps, while the clear eiiiuent overflows into the annular trough 23 and thence to outlet pipe 24..

My complete invention contemplates the utilization of the nickel cyanide for the manufacture of hydrocyanic acid, with the production of a nickel sulfate solution which is returned to the water-treating process. If the hydrocyanic acid is,V to be manufactured in or near the byproduct coke plant producing the cooler Water, the slurry from the Dorr thickener may be used without further treatment. If the hydrocyanic acid is to be made in a separate plant and at some distance from the point of treating the cooler water, it will be desirable to filter the slurry in order to avoid transportation of excessive amounts of water. In the apparatus shown in Figure 1 the slurry is pumped by pump 25 through pipe 26 into lter 2l, the filtrate passing out through pipe 28. The filter cake is then transported to the hydrocyanic acid plant.

The essential features of the hydrocyanic acid plant are shown in Figure 2. Weighed amounts of the nickel cyanide filter cake are placed in tank 29 to which is added sufjcient water to make a thick slurry which is pumped by pump 3S through pipe 3| to the decomposer 32. Into this sulfuric acid is pumped or blown from tank 33 through pipe 34, the amount of acid required being usually about twice the equivalent of the amount of nickel in the contents of the decomposer. Steam is blown into the decomposer through pipe 35 and indirect steam heating may also be employed.

The hydrocyanic acid and water vapor pass upward through the column 36 which may consist of several bubbling sections, the top sections preferably beingequipped with cooling coils 3l supplied with water from pipe 38. Much of the water v`apor is caught in this column and returned to the decomposer. The concentrated hydrocyanic acid gas then passes through a dephlegmator 39 containing cooling coils, also supplied with water from pipe 38. This dephlegmator removes most ofthe remaining water. The hydrocyanic acid gas then passes through pipe 4D into condenser 4| containing cooling coils or sections cooled with refrigerated brine which enters at 42 and leaves at 43. Liquid hydrocyanic acid passesout through pipe 44 and is collected in suitable containers.

After completion of rdecomposition of the nickel cyanide and expulsion of the hydrocyanic acid so far as practicable, valve 45 is closed and valve 46 opened and the decomposer and column are thoroughly purgedV with steam which passed out through pipe 4l carrying any residual hydrocyanic acidi This may be scrubbed out in a small water-scrubber and precipitated with lime and nickel sulphate or recovered in some other manner.

The acid nickel sulphate liquor, free from hydrocyanic acid, is removed from the decomposer by opening valve 43 and allowing the liquor to run into tank 49. As the nickel cyanide filter cake usually contains calcium carbonate and possibly calcium sulphate, the acid nickel sulphate liquor may contain a substantial amount of insoluble calcium sulphate. It is usually` desirable to remove this by sedimentation or filtra` tion before transferring the nickel sulfate liquor to tank I3 in Figure l, where it is used in the recovery of additional nickel cyanide.

The gas cooler water usually contains rnaphthalene, some of which may contaminate the nickel cyanide precipitate. It is desirable to remove such naphthalene from the filter cake so as to avoid contamination of the hydrocyanic acid and possible stoppage of parts of the decomposing apparatus. For this purpose the filter cake may be thoroughly steamed before removing it from the filter 2l, or else it may be steamed in tank 29.

Unless the aeration treatment of the gas cooler water for removal of hydrogen sulfide is supplemented by treatment with oxidizing agents or other reactive substances added to the water simultaneously with, or immediately following,

aeration, the nickel cyanide precipitate will contain small amounts of nickel sulfide which will be converted into hydrogen sulfide in the decomposer, For many purposes this will do no harm; but Where it is desired to completely eliminate all traces of hydrogen sulfide, I have found that this may be effectively accomplished by adding an oxidizing agent, such as a permanganatev or bichromate, to the nickel cyanide slurry in decomposer 32 before or simultaneously with the addition of the sulfuric acid. Alternatively the solution of the oxidizing agent may be added to the top section or one of the upper sections of column 36, the addition being made from tank 5l) through pipe 5l and valve 52. Best results are obtained by using twice as much bichromate or permanganate as is theoretically required to oxidize the hydrogen sulfide evolved; but as very small amounts of nickel sulfide are present, the cost of this excess of oxidizing agent is inconsiderable.

Instead of removing hydrogen sulde'from the gas cooler water by aeration or other means` preliminary to precipitation of nickel cyanide, I may permit such hydrogen sulfide to remain and be precipitated as nickel sulfide simultaneously with the precipitation of the nickel cyanide. In the subsequent treatment of the precipitate with sulphuric acid, advantage may be taken of the fact that the rate of decomposition of nickel sulphide byu acidsis much slower than that of nickel cyanide. For example, samples of unaerated gas cooler water containing 0.15 tol 0.17 gram of hydrogen sulphide per liter were precipitated with nickel sulphate and lime and the precipitate was filtered off and distilled with an excess of normal sulphuric` acid. After ten minutes, 85-93% of the hydrocyanic acid had distilled over; but thiswas accompaniedby only 0.5-1.5% of the .originalhydrogensulphide.

' ing of the contents of the decomposer 32 may be stcppedwhen approximately 90% of the hydrocyanicacid is expelled and the contents may be transferred to another vessel where they may be heated for a longer perior until all of the nickel.

sulphide is decomposed. The gases from this second vessel maybe burned to prevent danger f from the hydrocyanic acid, or elsemay be conveyed to the stream of coke oven gas entering the final `cooler where the hydrocyanic acid will be absorbed. f

Instead of separating the hydrogen sulphide as just described, I may employ methods similar to bthose stated in copending application Serial No.

*y y10,156; vizz- First, I may scrub the gases evolved from the decomposer with a portion of the nickel cyanide slurry, obtained in the precipitation process, the

action being similar to that of zinc cyanide as described in the copending application.

Second, I may yseparate hydrogen sulphide gas from the liquid hydrocyanic acid produced in f the condensing apparatus, taking advantage of the-fact that the temperature of condensation of 'hydrocyanic acid gas is much higher vthan that of hydrogen sulphide.' The apparatus preferred forv carrying out these methods has been described in the copending application.

j As stated in the forego-ing descriptions, I employ lime as the alkaline precipitating agent in 'conjunction With nickel sulfate for the production of nickel cyanide,'the accurate control of 1 Ythe hydrogen ion concentration being essential tothe success of the process. Other alkaline agents may be substitutedfor lime provided they do not form nickel compounds which do not react or react too slowly with hydrocyanic acid.

VForY example, I have successfully used sodium hydroxide, magnesia and ammonia. The substitution of ammonia for lime may be economical in many ley-products coke plants.

Since hydrocyanic acid is a very Weak acid, any of a large number of Water soluble acids can ybe used to regenerate hydrocyanic acid from ,the

insoluble cyanide. For example, hydrochloric acid may be used and sulfur dioxide may also be employed to decompose the nickel cyanide. The

`latter yis Vparticularly advantageous because of its well yknown inuence in retarding or preventing the decomposition of hydrocyanic acid.

This invention is not confined to vthe specific Y form of apparatus shown because other unitsof equal function may be substituted for any of I the units of this apparatus. The operating conditions such ras temperature, pressure, rate of flow and relative proportion of reagents, may

l also be varied within the scope of the following claims.

I claim: Y r1. The process of recovering hydrocyanic acid Y from gases containing it which comprises scrubi bing the gases with Water to form a dilute aqueous,

. solution of f the hydrocyanic acid, treating it 65 l cyanide from an approximately neutral solution,

with a soluble nickel salt to precipitate nickel separating the nickel cyanide thus precipitated from the Water, and treating it with an acid which will regenerate the vhydrocyanic acid.

2; 'I he process of recovering hydrocyanic acid 4from gases containingy it which comprises scrub- .bing the gases with Water to form a dilute aqueous solution of the hydrocyanic acid, treating it with a soluble nickel salt at a pH of 6.8 to 8.2

to precipitate nickel cyanide, separating the from ygases containing it which comprises scrubbing the gases with water to form a dilute aqueous solution of the hydrocyanic acid, treating it With a soluble nickel salt and sufficient alkali to adjust the pH to 6.8 to 8.2 in order to precipitate nickel cyanide, separating the nickel cyanide from the water and treating it with sulfuric acid to regenerate the hydrocyanic acid.

4. The process of recovering hydrocyanic acid from gases containing it which comprises scrubbing the gases With Water to form a dilute aqueous solution of the hydrocyanic acid, treating it with a soluble nickel salt and suflicient lime to adjust the pH to 6.8 to 8.2 in order to precipitate nickel cyanide, separating the nickel cyanide from the Water and treating it with sulfuric acid to regenerate the hydrocyanic acid.

5. The process of recovering hydrocyanic acid from gases containing it which comprises scrubbing the gases with Water to form a dilute aqueous solution of the hydrocyanic acid, treating it with nickel sulfate and adding milk of lime to adjust the pI-I to 6.78 to 8.2,- thus precipitating the nickel cyanide, separating the nickel cyanide from the water, treating it with sulfuric acid to regenerate the hydrocyanic acid, and condensing the hydrocyanic acid.

6. The process of recovering hydrocyanic acid from gases which also contain hydrogen sulfide which comprises scrubbing the gases with Water to form a dilute aqueous. solution of hydrocyanic acid, aerating said solution to remove most of the absorbed hydrogen sulfide, treating said solution with a soluble nickel salt at a pH of from 6.8 to 8.2, separating the precipitated nickel cyanide fro-m the Water, and treating it with an acid which Will regenerate the hydrocyanic acid.

7. The process of recovering hydrocyanic acid from gases which also contain hydrogen sulfide which comprises scrubbing the gases with water to form a dilute aqueous solution of hydrocyanic acid, treating it with a soluble nickel salt at a pH of 6.8 to 8.2 to precipitate nickel cyanide and nickel sulfide, separating the precipitate from the water and treating it with an acid which will regenerate the hydrocyanic acid, said acid treatment being discontinued when the bulk of the hydrocyanic acid has been liberated and before the nickel sulfide is substantially affected,

8. The process of recovering hydrocyanic acid from gases which also contain hydrogen sulfide Which comprises scrubbing the gases with water to form a dilute aqueous solution of hydrocyanic acid, treating it with a soluble nickel salt and suicient lime to adjust the pH to from 6.8 to 8.2,

separating the precipitated nickel cyanide and FREDERICK W. SPERR, JR. 

